This invention relates to methods of expressing proteins in recombinant hosts and more particularly to expressing in microbial hosts heterologous eukaryotic proteins that require formation of disulfide bridges for biological activity.
A variety of proteins are known which have commercial and medical application and which are characterized in having a complex molecular structure stabilized by disulfide bridging. One such class of the proteins, the disintegrins, include a class of cysteine-rich proteins that are the most potent known soluble ligands of integrins (Gould, Polokoff et al. 1990; Niewiarowski, McLane et al. 1994). The tri-peptide motif RGD (Arg-Gly-Asp) is conserved in most monomeric disintegrins (Niewiarowski, McLane et al. 1994). The RGD sequence is at the tip of a flexible loop, the integrin-binding loop, stabilized by disulfide bonds and protruding from the main body of the peptide chain. Disintegrins bind to the fibrinogen receptor αIIbβ3, which results in the inhibition of fibrinogen-dependent platelet aggregation (Savage, Marzec et al. 1990). Except for barbourin, a KGD-containing disintegrin, which is a relatively specific ligand for αIIbβ3 integrin (Scarborough, Rose et al. 1991), other disintegrins are rather nonspecific and can block or disturb the signaling pathways associated with the function of other β3 integrins, as well as β1 integrins (McLane, Marcinkiewicz et al. 1998).
Contortrostatin (CN) is the disintegrin isolated from Agkistrodon contortrix contortrix (southern copperhead) venom (Trikha, Rote et al. 1994). CN displays the classical RGD motif in its integrin-binding loop. Unlike other monomeric disintegrins, CN is a homodimer with a molecular mass (Mr) of 13,505 for the intact molecule and 6,750 for the reduced chains as shown by mass spectrometry (Trikha, Rote et al. 1994).
Receptors of CN identified so far include integrins αIIbβ3, αvβ3, αvβ5, and α5β1 (Trikha, De Clerck et al. 1994; Trikha, Rote et al. 1994; Thou, Nakada et al. 1999; Zhou, Nakada et al. 2000). Interactions between CN and integrins are all RGD-dependent. As an anti-cancer agent, CN has shown to be a powerful anti-angiogenic and anti-metastatic molecule in in vitro cell-based functional assays and in vivo animal models (Trikha, De Clerck et al. 1994; Trikha, Rote et al. 1994; Schmitmeier, Markland et al. 2000; Zhou, Hu et al. 2000; Markland, Shieh et al. 2001; Swenson, Costa et al. 2004). CN also has the ability to directly engage tumor cells and suppress their growth in a cytostatic manner (Trikha, De Clerck et al. 1994; Trikha, Rote et al. 1994; Schmitmeier, Markland et al. 2000). The antitumoral activity of CN is based on its high affinity interaction with integrins α5β1, αvβ3 and αvβ5 on both cancer cells and newly growing vascular endothelial cells (Trikha, De Clerck et al. 1994; Zhou, Nakada et al. 1999; Zhou, Nakada et al. 2000; Zhou, Sherwin et al. 2000). This diverse mechanism of action provides CN with a distinct advantage over many antiangiogenic agents that only block a single angiogenic pathway and/or do not directly target tumor cells.
CN full-length DNA precursor has been cloned and sequenced (Zhou, Hu et al. 2000). CN is produced in the snake venom gland as a multidomain precursor of 2027 bp having a 1449 bp open reading frame (encoding proprotein, metalloproteinase and disintegrin domains), which is proteolytically processed, possibly autocatalytically, to generate mature CN. The CN disintegrin domain encodes 65 amino acids with a molecular weight equal to that of the CN subunit. The CN full-length precursor mRNA sequence can be accessed in the GeneBank database using accession number: AF212305. The nucleotide sequence encoding the 65 amino acid disintegrin domain of CN represents the segment from 1339 to 1533 in the mRNA. Plasmids encoding the CN full-length gene have been described (Zhou, Hu et al. 2000) and are available from the laboratory of Francis S. Markland at University of Southern California.
Structurally, CN is a cysteine-rich protein (10 cysteines per monomer) that displays no secondary structure and, like other disintegrins, has a complex folding pattern that relies on multiple disulfide bonds (four intrachain and two interchain disulfide bonds) to stabilize its tertiary structure (Thou, Hu et al. 2000). By folding in a compact structure locked by multiple disulfide bonds, CN, like many other venom proteins, has a survival advantage, being less susceptible to a proteolytic attack and better equipped to survive in the harsher extracellular microenvironment. Its highly cross-linked structure and unique biological activity are barriers to producing biologically functional CN (or other disintegrin domain protein) using a recombinant expression system. A further difficulty is that the CN disintegrin domain of the multidomain precursor, from which dimeric CN is derived, displays no secondary structure, a feature that is known to facilitate the proper folding in most nascent proteins (Moiseeva, Swenson et al. 2002). The crystal structure of native CN has not been elucidated. CN's folding pattern is presumably as complex as other viperid dimeric disintegrins that have been studied (Calvete, Jurgens et al. 2000; Bilgrami, Tomar et al. 2004). Attempts to express snake venom disintegrins such as CN as functional native conformers and at a high level of expression suitable for mass production in eukaryotic and prokaryotic systems have been so far disappointing (e.g., see (Moura-da-Silva, Linica et al. 1999).